well interesting scenario, aside for the years and years together, this has happened to me. I do not regret what i did at the time and would do it all over again. When i walked in on them i simply said "Well looks like you found what your looking for so you can keep it", looked at the guy and said "Good luck to ya, she did this to me, and she will do it to you". Later i heard that four months later they were married.....

Now, if this is something that was out of the blue and you had no idea, then say good bye, the trust has been broken and can not be repaired. If you have both talked about spicing things up and bringing someone else in the bedroom then thats something you have to think about happening more often, the possibility of someone sleeping around behind y our back doesnt give me the warm and fuzzy feeling that i want out of relationship....